This invention relates to a process for the nuclear chlorination of alkylbenzenes and, in particular, for the directed nuclear chlorination of xylene, especially o-xylene and m-xylene.
The chemical reaction of chlorine with alkylbenzenes, such as xylene or toluene, or the like to prepare nuclear substituted chloro-compounds such as monochloroxylene or monochlorotoluene, is well known and of considerable commercial importance. Such reactions are generally carried out in the presence of a chlorination catalyst such as antimony chloride, ferric chloride, aluminum chloride, and the like. The usual products of such reactions are a mixture of various mono-chlorinated and/or polychlorinated compounds and various positional isomers of these. For example, in the liquid phase substitution-chlorination of toluene, by reaction of chlorine and toluene, to form monochlorotoluene, the usual product is a mixture of orthochlorotoluene and parachlorotoluene which may, in addition, contain varying amounts of other chlorinated products. Of the major reaction products, that is orthochlorotoluene and parachlorotoluene, the latter is the most commercially valuable. In the past, considerable effort has been expended in attempts to direct the chlorination reaction in such a manner as to lower the ratio of orthochlorotoluene to parachlorotoluene, that is, to discover reaction conditions under which the formation of parachlorotoluene is favored. Similarly, in the nuclear chlorination of xylene, the reaction product is typically a mixture of chloroxylene compounds and/or positional isomers. Thus, for example, in the chlorination of o-xylene, to form monochloroxylene, the usual product is a mixture of 4-chloro-o-xylene and 3-chloro-o-xylene. When the reaction of chlorine and o-xylene is carried out in the presence of a ring chlorination catalyst, such as Fe, FeCl.sub.3, SbCl.sub.5, or the like, the monochlorinated reaction product is typically characterized by a ratio of 4-chloro-o-xylene:3-chloro-o-xylene of less than about 1.5:1. For some purposes the 4-chloro isomer is preferred over the 3-chloro-isomer, for example, as an intermediate in the preparation of insecticides. For this reason, efforts have been expended in attempts to direct the chlorination of o-xylene in such a manner as to increase the ratio of 4-chloro-o-xylene:3-chloro-o-xylene produced, that is, to discover reaction conditions or methods whereby the formation of 4-chloro-o-xylene is increased.
It is known from U.S. Pat. No. 1,946,040 that when alkylbenzenes are reacted with chlorine, the yield of parachlorinated product is improved with the aid of a mixed catalyst comprising sulfur and antimony trichloride and, optionally, iron or lead. In British Pat. No. 1,153,746 (1969) it is disclosed that in the chlorination of toluene in the presence of a ring chlorination catalyst, such as ferric chloride, antimony chloride, and the like, the ratio of orthochloro to parachloro isomers produced may be lowered by the presence of an organic sulfur compound such as thiophene, hexadecylmercaptan, dibenzothiophene or the like. Furthermore, in British Pat. No. 1,163,927 (1969) it is disclosed that the proportion of parachlorotoluene produced may be improved when toluene is chlorinated in the presence of elemental sulfur or an inorganic sulfur compound and a ring-chlorination catalyst such as ferric chloride, aluminum chloride, antimony chloride, zinc chloride, iodine, molybdenum chloride, stannous chloride, zirconium tetrachloride or boron trifluoride. In U.S. Pat. No. 3,226,447, issued Dec. 28, 1965 to Bing et al, it is disclosed that in the substitution-chlorination of benzene and toluene, the ratio of ortho isomer to para isomer in the chlorinated product may be lowered when the reaction is carried out in the presence of an iron, aluminum or antimony halide catalyst and a co-catalyst which is an organic sulfur compound wherein the sulfur is divalent. Examples of such co-catalyst include various mercaptans, mercapto-aliphatic carboxylic acids, aliphatic thiocarboxylic acids, alkyl sulfides, alkyl disulfides, thiophenols, aryl sulfides, aryl disulfides and the like containing divalent sulfur. The use of such co-catalysts in the chlorination of toluene produces a product wherein the ratio of orthochlorotoluene to parachlorotoluene is 1.2, indicating a considerable improvement over the ortho to para isomer ratio achieved in the absence of the co-catalyst. However, it will be apparent that even a 1.2 ratio of ortho to para isomer represents a considerable economic disadvantage in the production of substantial amounts--greater than 50 percent of the monochlorotoluene mixture--of the unwanted ortho isomer. Thus, it will be apparent that a considerable commercial benefit is to be derived from a still further lowering of the ortho to para isomer ratio.
Still further improvements in the preparation of monochloroalkylbenzenes wherein para-chloro isomer production is favored, are disclosed in U.S. Pat. Nos. 4,031,142 and 4,031,147. U.S. Pat. No. 4,031,142 discloses a process for the preparation of nuclear chlorinated alkylbenzenes, such as monochlorotoluene which comprises reacting an alkyl-benzene, such as toluene, with chlorine in the presence of a Lewis acid catalyst and, as a co-catalyst, thianthrene. When alkylbenzenes are chlorinated in accordance with the process of U.S. Pat. No. 4,031,142, the formation of parachloro-isomers is favored. In accordance with U.S. Pat. No. 4,031,147, the formation of parachloroalkylbenzenes is favored when alkylbenzenes are reacted with chlorine in the presence of a Lewis acid catalyst and a co-catalyst comprising a thianthrene compound or mixture of thianthrene compounds characterized by the formula: ##STR2## where each n is 0 to 1, and each x is hydrogen or an electron-withdrawing substituent. Although the processes of the prior art provide improvements in the preparation of parachloroalkylbenzenes, it will be apparent that still further improvements, especially in the preparation of parachloroxylenes, would be desirable and of commercial benefit.
It is an object of the present invention to provide an improved process for the directed nuclear chlorination of aromatic compounds. It is a further object to provide a process for the directed nuclear chlorination of alkyl-benzenes, especially xylene, whereby the chlorinated product is characterized by a desirably high ratio of parachloro or 4-chloro-, to other monochloroisomers. It is a still further object to provide an improved para-directing co-catalyst for such processes. It is a still further object to provide a novel catalyst system based on a para-directing co-catalyst comprising a thianthrene compound or mixture of thanthrene compounds, having both electron-withdrawing substituents and electron-donating substituents on the nucleus thereof.
The thianthrene compounds employed as para-directing co-catalysts in accordance with this invention are described hereinbelow in accordance with the current Chemical Abstracts system whereby the numbering of ring positions is as follows: ##STR3##